Molded plastic closures for containers have become increasingly popular for closing and sealing the contents of containers, such as for use with carbonated and non-carbonated beverages, condiments, and other food products, as well as for use on non-food products, such as motor oil. While various plastic closure constructions have been known for a number of years, special design considerations must be made in order for the desired sealing to be effected, and to facilitate high-speed manufacture and application of such closures.
In order to achieve the desired sealing performance, a number of heretofore known closure constructions have been of a composite nature, including a relatively rigid outer plastic shell, and a relatively pliable and soft inner sealing liner. One such construction, formed in accordance with commonly-assigned U.S. Pat. No. 4,343,754, U.S. Pat. No. 4,378,893, U.S. Pat. No. 4,407,422, and U.S. Pat. No. 4,497,795, all incorporated herein by reference, have proven to be highly effective for use on containers, including containers having carbonated or otherwise pressurized contents. The desired sealing performance is achieved by configuring this closure construction to include a relatively rigid and strong threaded outer closure cap, with an in situ formed liner positioned adjacent a top wall portion of the cap. Closures in accordance with these patents are configured to form a so-called top/side seal, in that the liner can sealingly engage both a generally upwardly facing surface of an associated container, as well as a generally outwardly facing, side surface of the container.
While it will be appreciated that eliminating the inner sealing liner of such closure construction would decrease the attendant cost of its manufacturing use, it has heretofore proven difficult to provide a closure construction exhibiting the requisite strength, while at the same time including a sealing arrangement exhibiting sufficient flexibility and conformability to achieve the desired sealing engagement with associated containers. Development of a linerless, one-piece closure construction suitable for use on containers having pressurized contents has heretofore met with limited success.
Another design consideration which must be addressed concerns application of closures. Experience has shown that a typical threaded closure can exhibit a tendency to skew during application, which is believed to result from the asymmetrical nature of the typical single helical thread formation within the closure. Specifically, the side of the closure at which the closure thread terminates at or near the top wall portion of the closure exhibits a relatively high degree of rigidity and strength. In contrast,. the opposite side of a single thread closure, whereat the thread is disposed one-half thread pitch further from the top wall portion, is the so-called "weak side" of the closure, exhibiting relatively less rigidity and strength (this phenomenon is not so pronounced with multi-thread closures, which tend to balance the application forces due to the symmetry of the multiple threads).
As a consequence, application of a single thread closure ca result in skewing of the closure as the "strong side" engages the associated container more securely and firmly than the "weak side" of the closure. Experience has shown that skewing of the closure which results from this effect can adversely effect the sealing performance of the closure, since its sealing element or elements may not be firmly and squarely seated on the container. This skewing can adversely affect both proper seating of a top seal as well as proper seating of a side seal.
The present closure is particularly configured for enhanced sealing performance while facilitating application to a container.